Diabetes is a group of metabolic diseases characterized by high blood glucose, and at present there is no radical cure to it yet. The treatment of diabetes needs frequently monitoring glucose to control the blood glucose level. The conventional invasive blood sampling method has obvious defects, and, it causes wound and pain to the patient during measurement, and is inconvenient for continuous monitoring. Non-invasive blood glucose monitoring technology overcomes the drawbacks of the conventional method, which can effectively meet the demand of diabetic patients for real-time and frequent monitoring of blood glucose concentration. Non-invasive method is the developing direction of blood glucose detection technology. Optical method is the most studied non-invasive glucose monitoring method. However, due to numerous interferences and individual differences, most of the them are still in a laboratory research stage.
Since Lukaski put forward using bioelectrical impedance analysis (BIA) to measure body components of human in 1985, researchers have used BIA method to differentiate components of human body, including fats, muscles, minerals, and aqueous substances, etc. Fatty substances and non-fatty substances have different current conducting properties, so that different tissues and organs have different impedance characteristics. The Inbody series human body composition analyzers developed by a Korean listed company measure the balance condition of human body components at high accuracy. It employs a multi-frequency bioelectrical impedance analysis method in different segment of body.
Based on the biological impedance technique, some progress has been made in the research on non-invasive blood glucose monitoring based on impedance spectroscopy (IS) method. Most of researches on impedance spectroscopy focus on frequencies within 0-50 kHz and below 10 MHz. Harry Richardson Elden, et al., (WO1999039627 A1) from USA measures impedance amplitude and phase of the skin of human body at specific frequency points (20 kHz, 500 kHz) and utilizes a linear combination of impedance and phase to predict blood glucose; a research team led by Kiseok Song in Korea incorporates impedance spectroscopy and infrared spectrometry to carry out non-invasive blood glucose tests, with a frequency range of 10 kHz-76 kHz. Researches at a low frequency are relatively easy since there is no need to concern the impact of radio frequency transmission, high-frequency noise interference, and electrode polarization, etc.; however, at low frequency, the current bypasses cells and flows through the extracellular fluid, influencing the result of non-invasive blood glucose monitoring.
A research team led by Caduff A (US2013/0211204A1, U.S. Pat. No. 7,693,561B2) in Swiss has found that there is an apparent relation between blood glucose concentration and impedance value in higher frequency bands. The research team designed an impedance measurement system within 30-60 MHz frequency, and studied the correlation between obtained impedance information and blood glucose. The high-frequency method has a high requirement for stability of tissue characteristics. However, owing to a severe difference in skin thickness and tissues among different individuals, it is hard to attain a satisfactory result if the high-frequency method is used alone. In the subsequent researches, the research team began to employ a multi-sensor and multi-parameter measurement method to improve the accuracy, including electrodes in high frequency band, intermediate frequency band and low frequency band, temperature, humidity and optical sensors. The electrodes included a vertical bar electrode and annular electrodes around the vertical bar electrode, and the distances between the vertical bar electrode and the surrounding annular electrodes were 0.3 mm, 1.5 mm, and 4 mm respectively; the wavelengths used by the optical sensor were 550/660/880 nm. The electrodes used by the research team had a strip shape at one end and an annular shape at the other end, and the electrodes operating at different frequencies were close to each other and were surrounded by identical grounding wires; therefore, interference existed among the electrodes; since two electrodes operating at the same frequency were very close to each other, the penetration depth in the tissue was shallow; the temperature and humidity sensors were directly attached to the skin, causing humidity saturation easily and influencing temperature and humidity test.